Various types of biometric systems are used more and more in order to provide for increased security for accessing an electronic device, thereby providing an enhanced user convenience. In particular fingerprint sensors have been successfully integrated in such devices, for example, thanks to their small form factor, high performance and user acceptance. Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc.), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important issues.
To save cost and valuable surface space, there is an effort towards smaller and smaller fingerprint sensors. In contrast to “all at once” fingerprint scanners, which capture an image of an entire fingerprint at the same time, such smaller fingerprint sensors may be substantially smaller than the user's fingerprint. By imaging only a portion of a fingerprint at any given time, the size and cost of a partial fingerprint sensor can be made considerably smaller and cheaper than that of a full fingerprint sensor. A plurality of the smaller fingerprint image portions are then combined into a complete fingerprint image, e.g. by means of feature extraction.
For allowing the smaller fingerprint image portions to be combined with each other, it is desirable if each of the fingerprint image portions has a sufficient level of quality. This typically place constrains on how the finger is contacting the fingerprint sensor. Due to various human factors issues, it is not so easy for the user to know exactly how to position his finger in relation to the fingerprint sensor, and different users will place their fingers in different ways. In order to account for this type of variation, modern partial fingerprint sensors often incorporate mechanical means for guiding the user (e.g. a depression) and/or finger position sensors to determine the finger position in relation to the fingerprint sensor.
An example of a fingerprint system comprising such a finger position sensor is disclosed in U.S. Pat. No. 8,077,935, the finger position sensor including a position pickup plate and multiple position drive plates. During operation, the drive plates are energized sequentially with signal bursts. In the case where a finger is in contact or near contact with the energized drive plate and the pickup plate, the signal burst is conducted through the bulk of the finger to the pickup plate. In the case where the finger is not in contact with the energized drive plate, the signal burst is conducted through air to the pickup plate, and a much smaller signal is detected. Thus, the sensed signal level indicates whether the finger is in contact with the energized drive plate and the pickup plate. By analyzing the detected signals from all of the drive plates, the position of the finger end can be determined and the user may be provided with an indication of an actual finger placement and a desired finger placement of the finger in relation to the fingerprint sensor.
Even though the fingerprint sensing system according to U.S. Pat. No. 8,077,935 provides for improvements in relation to the use of “smaller” fingerprint sensors, the additional inclusion of the finger position sensor limits the mounting possibilities in relation to a portable electronic device, such as a mobile phone, where the available real-estate is highly limited. In addition, the suggested guidance does not take into account the fact that different users have different levels of experience of using fingerprint sensors and thus need different level of guidance.
Further attention is drawn to US 2010/0303311 A1, guiding a user as how to positioning his finger in relation to a fingerprint sensor. The guiding is directly related to a current position of the finger in relation to the fingerprint sensor.